THIS invention relates to a catalyst particle comprising a titanium dioxide rutile nano support structure loaded with metal nanoparticles that is catalytically active for multiple chemical reactions across a wide range of temperatures after exposure to high temperatures for prolonged periods of time. In particular, but not exclusively, this invention relates to a catalyst particle comprising a support structure of titanium dioxide rutile nanorods extending radially from a central point, wherein the nanorods are loaded with gold (Au) nanoparticles, the catalyst being catalytically active from below room temperature to high temperatures after exposure to temperatures in excess of 550° C. for prolonged periods of time.
Supported Au nanocatalysts are known to have very high activities in a number of important industrial reactions, including oxidation of CO and hydrocarbons, reduction of NOx, water-gas-shift reaction, H2O2 production from H2 and O2, removal of CO from hydrogen steams, and selective epoxidation as well as oxidations. Further known uses of nanogold catalysts include oxidation of propylene to propylene oxide, combating pollution as welt as prolonging the life of hydrogen fuel cells. The remarkable performance of gold-based catalysts in CO oxidation spurred scientists to test these catalysts in other oxidation reactions, such as the epoxidation of alkenes, oxidative destruction of hydrochlorides, and oxidation of CH4. Many of these aforementioned reactions are currently catalysed by metals from the platinum group metals (PGM's).
Haruta et al. reported (M. Haruta, N. Yamada, T. Kobayashi and S Iijima, J. Catal., 115, (1989), 301) that supported nanogold is able to catalyse the oxidation of carbon monoxide at temperatures as low as −70° C. Au catalysts therefore have the potential to catalyse other reactions which are currently catalysed using PGM's, such as in autocatalysts. Nanogold catalysts are able to catalyse exhaust gas streams at much lower temperatures than current PGM based catalysts. When the CO oxidation reaction is considered the rate of oxidation of CO for Au catalysts is more than one order of magnitude higher than those for similarly prepared platinum catalysts. In other words, Au based autocatalysts are able to catalyse exhaust emissions from very low temperatures avoiding higher light off temperatures suffered by current PGM based catalysts. This exceedingly high activity of Au catalysts for CO oxidation has been a subject of great interest in the catalysis community.
The other feature of the low temperature activity is the use of Au catalysts in CO filters for gas masks and air scrubbers. In this application a catalyst is required that does not lose activity with time and is able to operate at high efficiency at low temperatures.
Such activity is unique to supported Au nanoparticles, however it only occurs if the supported Au nanoparticles are smaller than 8 nm in size, with optimal activity attained at around 5 nm or smaller.
Furthermore, both the activity and the selectivity of the catalyst is dependent on the Au particle size as well as the support used to hold the Au. However, the direct applications of supported Au nanocatalysts to the above industrial processes, including autocatalysts, have been hampered by the instability of the Au nanocatalysts against sintering under extreme reaction conditions and also under standard conditions over prolonged time periods.
Accordingly, ultra stable supported Au nanocatalysts that are particularly impervious to high temperature treatments in any atmosphere are extremely desirable to industrial applications because most of the aforementioned reactions proceed at relatively high temperatures. In addition, in the case of autocatalyst applications, catalyst activity also needs to be maintained at low temperatures thereby avoiding pollution created while the engine is cold, before the current Pt based catalysts reach their light-off temperature. In addition, in the case of filter masks the catalyst needs to remain active after long periods of shelf life—often in non-ideal conditions.
The literature indicates that current supported Au based catalysts intended for high temperature applications, i.e. over 450° C., are only calcined for a short amount of time and then tested for catalytic activity. Further, many of the catalysts are bi- or tri-metallic catalysts with Au forming the minor component and PGM's the major.
Two important, but distinctly different, factors have been suggested to be important for the control of the activity of Au catalysts; firstly Au particle size, and secondly support effects.
The correlation between activity and Au particle size has been clearly demonstrated for Au nanoparticles supported on reducible metal oxides and it is generally accepted that the high catalytic activity of supported Au catalysts in low temperature CO oxidation can be accredited to the presence of small Au crystallites, which are stabilized by the support and where the strong interaction with the support helps to create a favourable electronic environment to promote the activity of the Au.
Furthermore, the support interface has also been shown to play an important role in the catalysis. In order to try and stabilise gold nanoparticles for high temperature applications various supports, including TiO2, Al2O3, CeO2, ZnO, SiO2, ZrO2, and Co3O4 as well as combinations of these, have been used to test Au for the catalytic oxidation of CO. Of all of these, TiO2 in the anatase phase has been found to be one of the most active, and has been extensively used for a number of years as it is known to be highly active with Au for the oxidation of CO. Titania is in a class of supports known as active supports due its ability to be easily reduced and facilitate the transfer of oxygen between the support and the Au. This is further improved by the effect of its isoelectric point on the deposition of Au nanoparticles to form strong bonds with the TiO2 support.
Anatase has long been the preferred phase of TiO2 when considering possible supports for various reactions due to its large surface area when compared to rutile's 7.2 m2/g average surface area. However, the limiting factor for both anatase and P25 is the conversion of anatase to the thermodynamically preferred rutile phase which results in a massive loss of catalytic activity. This conversion is driven by temperature and is affected by the presence of the Au on the support.
A significant amount of research has been conducted on attempting to stabilise Au nanoparticles for higher temperature applications such as automotive catalysts.
An article by Mellor et. al. (Mellor J. R, Palazov A. N, Grigorova B. S, Greyling J. F, Reddy K, Letsoalo M. P, Marsh J. H, (2002) Catal. Today 72:145) a catalyst containing Au on cobalt oxide particles supported on a mixture of zirconia-based ceria, zirconia and titania was able to survive 157 hours at 500 deg C. However, a large loss of activity and a large loss of support surface area were reported.
In work developed by Seker and Gulari, Au—Al2O3 catalysts were able to survive pre-treatments at 600 deg C. in air for 24 hours followed by several cycles of 150-500 deg C. The catalysts were then kept at 500 deg C. for 12 hours and showed high activities for NO conversion. However, NO conversion is less sensitive to Au particle size changes compared to the CO oxidation reaction implying the catalyst may have undergone deactivation for the CO oxidation reaction while still remaining active for NO conversion. No information was provided as to the catalyst's ability to oxidize CO. Much like the Mellor catalyst discussed above, the temperatures that the catalysts were exposed to were not significantly high when the duration of exposure was considered.
EP 1 043 059 to Toyota Jidosha Kabushiki Kaisha describes a catalyst containing complex gold oxides of the form Au2Sr5O6. In this catalyst the Au is entirely ionic and is trapped in the oxide lattice. The Toyota catalyst was tested to 800° C. for 5 hours with only a small decrease in its ability to convert C3H6 as would be found in a typical exhaust gas stream. No data for the efficiency for CO oxidation was presented.
One of the most important reasons for the development of a nanogold catalyst is due to its ability, if the Au particles remain small enough, to facilitate reactions from ambient temperatures. The Toyota patent claims a T50 conversion at 345° C. for the fresh catalyst. This relatively high T53 value would somewhat negate the use of nanogold, as standard PGM based auto-catalysts are also active at this temperature. Therefore, this catalyst does not address the light off period at low temperatures.
Auto-catalysts are typically produced using a mixture of PGM's. Thus, for example, a standard type auto-catalyst will be a mixture of Pt—Pd supported on corderite along with CeO2. This type of catalyst has been shown to work very efficiently, but with the problem of light off at low temperatures.
U.S. Pat. No. 7,709,407 describe a method for producing a supported catalyst containing palladium-gold metal particles. The addition of Au is claimed to reduce the light off temperature and hence the catalyst is able to catalyse reactions from very low temperatures. However, if the catalyst reaches any significantly high temperature the stability of the Au nanoparticles must be called into question as Au supported on zeolites has been shown to be an unstable combination at high temperatures. No information on the durability of the catalyst after exposure to high temperatures was revealed. However, the catalyst was designed for diesel internal combustion engines where the exhaust gas temperatures are relatively mild when compared to gasoline engines.
If Au catalysts are to be used in applications above 400° C., such as in automotive catalysts, thermal stability of not only the Au nanoparticles but also the stability of the support is crucial for long term activity. It has been reported that, in a European driving cycle, temperatures average between 80-450 deg C., while in the extra urban part of the cycle average temperatures of 200-450 deg C. can be expected. Some Au catalysts may cope with these temperatures, however at certain times during the cycle temperatures may reach well over 500 deg C. and enter a thermal region that current Au based catalyst cannot operate in. In addition, prolonged and repeated expose to such temperatures will inevitably deactivate the Au.
Therefore, there remains a need for a catalyst that can meet the requirements of both thermal stability and durability, to be considered as a potential catalyst for reactions undertaken at temperatures from below ambient to above 550° C. for prolonged periods of time.
There remains a further need for an Au catalyst that can meet the requirements of both thermal stability and durability, to be considered as a potential catalyst for reactions undertaken at temperatures from below ambient to above 550° C. for prolonged periods of time.